The present invention relates to a method for controlling characteristics of a semiconductor integrated circuit (IC), and more particularly to a method for controlling characteristics of a semiconductor device formed in the semiconductor IC by irradiating an X-ray on a surface of the semiconductor device.
A important characteristic of a semiconductor integrated circuit such as an IC having a number of insulated gate field effect transistors (IGFET's) formed therein, e.g. a gate threshold voltage (Vtx) of the IGFET is determined by a manufacturing process thereof. An impurity concentration on a surface of a silicon substrate, a property and a thickness of a gate oxide film and gate electrode material are important factors to determine Vtx and it is difficult to change Vtx after the semiconductor device has been fabricated. It is possible to control Vtx by additional diffusion or ion implantation to a gate area during the manufacturing process of the semiconductor device but this method cannot be applied after the formation of the electrode.
On the other hand, it has been attempted to adjust the characteristic of the FET by a gamma ray or X-ray but this technology has not yet been put into practice because the characteristic of the irradiated FET changes in a subsequent thermal process.
The present invention has been made in the above background and it is an object of the present invention to provide a method for controllably altering an important basic characteristic of an IC wafer, e.g. a gate threshold voltage Vtx of an IGFET after the manufacturing process thereof. It is another object of the present invention to bring the basic characteristic as close to a specified value as possible to provide an IC having a uniform characteristic. It is still another object of the present invention to adjust the basic characteristic which somewhat deviates from a permissible range of the specification by an X-ray irradiation to bring it into the permissible range in order to enhance a yield of the IC.
In addition, it is an object of a second embodiment of the present invention to shorten a cycle time for characterization in designing a new IC product in order to shorten a period from the IC circuit design to the supply of the IC to market place.
It is a further object of the present invention to provide a method for obtaining a large volume of data from a small number of test sample IC's for feeding back the information for reviewing the circuit design and the manufacturing process.